Epiphytic and endophytic members of the Enterobacteriaceae associated with healthy Eucalyptus trees

Abstract

Studies presented in this thesis, highlights the importance of determining whether members of the Enterobacteriaceae can be associated with plants as epi- and endophytes. In particular, whether the causal agent of blight and die-back of Eucalyptus can survive both epi- and endophytically onlin its host as well as in weeds grown in close proximity to these hosts. This knowledge allows one a better understanding of the etiology and epidemiology of this disease. Appropriate management strategies can now be provided and the impact of the disease lessened in the nursery environment Chapter One presents an evaluation of the potential importance of Enterobacteriaceae as epi- and endophytes on/in plants. Some information is known about the epi- and endophytes associated with economically important agricultural crops. This information is largely lacking for tree species, especially those grown for commercial forestry purposes. Many Enterobacteriaceae occur both epi- and endophytically onlin plants including Pantoea ananatis. This pathogen is known to occur epiphytically on weeds as well as on its hosts where under ideal environmental conditions it is capable of causing disease symptoms. As an endophyte, P. ananatis occurs in dune grass where it fixes nitrogen and in sweet potato where it is believed to protect the plant against fungal pathogens. Chapter Two analyses healthy leaves, both young and mature, removed from various clones of the hydrid, E. grandis x E. nitens, for the presence of bacterial epi- and endophytes. Enterobacteriaceae were also isolated and these included Pantoea spp. and Enterobacter spp. P. ananatis was isolated both epi- and endophytically onlin healthy Eucalyptus tissue as well as from leaves removed from weeds growing in close proximity to the diseased plants. This thesis clearly indicates that P. ananatis can occur both epi- and endophytically in healthy Eucalyptus tissue. The movement of planting material into new environments where bacterial blight and die-back does not occur should be restricted. Irrigation practices in nurseries should be reviewed to prevent the accumulation of water on the plant surface which will allow for entry of the pathogen into the host through natural openings. Another management strategy that must also be recommended is that stringent weed control be implemented in the nursery environment.